Attaining Melt Processing of Complementary Semiconducting Polymer Blends at 130 °C via Side-Chain Engineering

Complementary semiconducting polymer blends (c-SPBs) have been proposed and tested to achieve melt-processed high-performance organic field-effect transistors (OFETs). Prior to this study, melt processing requires temperatures as high as 180 °C. To implement this technique into low-cost and large-area thin-film manufacturing for flexible organic electronics, semiconducting materials meltable at temperatures tolerable by ubiquitous plastic substrates are still needed. We report here the design and melt processing of a c-SPB consisting of a matrix polymer (DPP-C5) and its fully conjugated analogue. By utilizing a siloxane-terminated alkyl chain and a branched alkyl chain as solubilizing groups, the matrix polymer DPP-C5 presents a melting temperature of 115 °C. The resulting c-SPB containing as low as 5% of the fully conjugated polymer could be melt-processed at 130 °C. The obtained OFET devices exhibit hole mobility approaching 1.0 cm²/(V s), threshold voltages below 5 V, and <i>I</i>_ON/<i>I</i>_OFF around 10⁵. This combination of efficient charge-carrier transport and considerably low processing temperatures bode well for melt processing of semiconducting polymer-based organic electronics

Conjugation-Break Spacers in Semiconducting Polymers: Impact on Polymer Processability and Charge Transport Properties

Conjugation-break spacers (CBSs) are intentionally introduced into the diketopyrrolopyrrole (DPP)-based polymer backbones. We reveal that the solution processability progressively increases with the percentage of CBSs, while charge mobility inversely varies to the CBS ratio. For instance, the polymer DPP-30 with solubility of ∼10 mg/mL in dichlorobenzene provides an average mobility over 1.4 cm² V^–1 s^–1, while DPP-0 exhibits an average mobility of 4.3 cm² V^–1 s^–1 with solubility of ∼3 mg/mL. This correlation provides a general guidance to design polymers with desired electronic performance and solution processability for large-scale roll-to-roll processing. Most encouraging, DPP-70 can be melt processed in air and provide hole mobilities up to 0.30 cm² V^–1s^–1, substantially higher value than their solution-processed counterparts about 0.1 cm² V^–1 s^–1. The mobility boost in melt-processed devices, together with completely eliminating the need to use toxic solvent in the processing, encourages to design melt-processable polymers for electronic devices

Complementary Semiconducting Polymer Blends for Efficient Charge Transport

Charge transport in polymeric thin films is a complicated process, which involves a multitude of coupled electronic events. Because of the growing appeal of semiconducting polymers in organic electronics, it makes the fundamental understanding of charge transport increasingly important. On the other hand, it urges the solution of the processability problem, frequently associated with high-performance polymers. In this study, we introduce complementary semiconducting polymer blends (<i>c</i>-SPBs), aiming to provide solutions for both the fundamental understanding of charge transport and the processability problem. The <i>c</i>-SPBs contain a highly crystalline matrix polymer with intentionally placed conjugation-break spacers (CBSs) along the polymer backbone, thus eliminating intrachain transport, and a tie chain polymer that is a fully conjugated polymer, restoring intrachain transport by connecting π-crystalline aggregates in the matrix polymer. The results show that the addition of as little as 1 wt % tie chain polymer into the matrix polymer induces a nearly 2 order of magnitude improvement in charge carrier mobility from ∼0.015 to 1.14 cm² V^–1 s^–1, accompanied by substantial lowering of activation energies from 100.1 to 64.6 meV. The morphological characterizations and electrical measurements confirm that tie chains are able to build the connectivity between crystalline aggregates, leading to efficient charge transport in the polymer blend films. Furthermore, this study suggests that <i>c</i>-SPBs can be a new platform for designing high-mobility electronic materials with enhanced solution processability for future organic electronics

Complementary Semiconducting Polymer Blends: The Influence of Conjugation-Break Spacer Length in Matrix Polymers

The concept of complementary semiconducting polymer blends (<i>c</i>-SPBs) for efficient charge transport was recently proposed and established by our group. In this study, we aim to reveal the influence of the length of conjugation-break spacers (CBSs) on charge transport properties of the matrix polymers and their corresponding complementary polymer blends. A series of 11 DPP-based semiconducting polymers DPP-C<i>m</i> (<i>m</i> = 2–12) that incorporate CBSs of 2–12 methylene units along the polymer backbones were prepared and characterized. The UV–vis spectra and the ultraviolet photoelectron spectroscopy (UPS) measurements show that the CBS length has marginal influence on the polymer absorption spectra, energy levels, and band gaps. It also has little impact on polymer decomposition temperatures. However, the CBS length has a profound influence on polymer phase transition and the heat of fusion. As for the melt transitions, an odd–even effect is observed from DPP-C2 to DPP-C7, in which polymers with even-numbered CBSs show higher melting points than their adjacent odd-numbered derivatives. The trend is opposite for heat of fusion. The polymers with odd-numbered CBSs exhibit larger heat of fusion, indicating higher ordering and crystallinity. The odd–even effect is also found in surface morphologies of the polymers by atomic force microscopy (AFM). The polymers with the even CBSs have a more interconnected feature that appear more fibrillar than the polymers with the odd linkages. As far as charge carrier mobility is concerned, the average number drops from 0.023 cm² V^–1 s^–1 to 7.9 × 10^–6 cm² V^–1 s^–1 as the CBS moves from C2 to C12. It is intriguing to observe that even-numbered polymers outperform the adjacent odd-numbered polymers, despite the fact that the latter show higher ordering and crystallinity in thin films. When these polymers are mixed with fully conjugated DPP-C0 (2 wt %, designated as tie chain polymer), the obtained <i>c</i>-SPBs witness a dramatic increase (2–4 orders of magnitude) in charge carrier mobility. Interestingly, the odd–even effect is not found for charge transport in the <i>c</i>-SPBs. This work reveals that the length of CBSs plays a significant role in charge transport properties of the matrix polymers and reconfirms that efficient charge transport properties of the <i>c</i>-SPB result from the interactions between matrix polymers and tie chain polymers. This begins to provide guidelines as to what spacer lengths may be utilized to offer the best balance between processing and charge transport properties

Symmetry Breaking in Side Chains Leading to Mixed Orientations and Improved Charge Transport in Isoindigo-<i>alt</i>-Bithiophene Based Polymer Thin Films

The selection of side chains is important in design of conjugated polymers. It not only affects their intrinsic physical properties, but also has an impact on thin film morphologies. Recent reports suggested that a face-on/edge-on bimodal orientation observed in polymer thin films may be responsible for a three-dimensional (3D) charge transport and leads to dramatically improved mobility in donor–acceptor based conjugated polymers. To achieve a bimodal orientation in thin films has been seldom explored from the aspect of molecular design. Here, we demonstrate a design strategy involving the use of asymmetric side chains that enables an isoindigo-based polymer to adopt a distinct bimodal orientation, confirmed by the grazing incidence X-ray diffraction. As a result, the polymer presents an average high mobility of 3.8 ± 0.7 cm² V^–1 s^–1 with a maximum value of 5.1 cm² V^–1 s^–1, in comparison with 0.47 and 0.51 cm² V^–1 s^–1 obtained from the two reference polymers. This study exemplifies a new strategy to develop the next generation polymers through understanding the property-structure relationship

Complementary Semiconducting Polymer Blends: Influence of Side Chains of Matrix Polymers

The concept of complementary semiconducting polymer blends (<i>c</i>-SPBs) has been recently proposed to achieve enhanced solution processability and/or melt-processing capability for organic electronics. In the previous study, we demonstrated the impact of conjugation-break spacers of matrix polymers. In the current work, we explore the influence of the side chains of the matrix polymer on the physical properties of the pure polymers and their corresponding <i>c</i>-SPBs, including electrical properties and phase transition behaviors. Six diketo­pyrrolopyrrole (DPP)-based polymers with pentamethylene conjugation-break spacers (CBSs) and various side chains, including branched-alkyl, triethylene glycol (TEG), and siloxane-terminated side chains, were synthesized and characterized. The UV–vis spectra show that the side chains have a noticeable impact on the intermolecular interactions in the solid states. In addition, side chains also have a significant influence on the thermal behaviors of the polymers. Polymers with asymmetric side chains attached to the same DPP unit exhibit lower melting points compared to the congeners with symmetric side chains. The polymer with both branched-alkyl and TEG side chain exhibits the lowest melting point of 104 °C. As for charge transport properties, polymers with branched-alkyl and/or siloxane-terminated side chains give hole mobilities on the same order of magnitude, whereas the polymers with TEG side chains exhibits much lower mobilities. When <i>c</i>-SPBs with a fully conjugated polymer with branched-alkyl side chains are concerned, the <i>c</i>-SPBs of all polymers, except for the polymer with only TEG side chains (TEG-DPP-C5), show hole mobilities 2 orders of magnitude higher than the corresponding pure matrix polymers. In contrast, TEG-DPP-C5 merely presents an improvement of 20 times, which resulted from the incompatibility of TEG side chains from the matrix polymer and the alkyl side chains from the tie chain polymer. These results provide new insights into structural design for semiconducting materials with both high performance and better processability

Understanding Interfacial Alignment in Solution Coated Conjugated Polymer Thin Films

Domain alignment in conjugated polymer thin films can significantly enhance charge carrier mobility. However, the alignment mechanism during meniscus-guided solution coating remains unclear. Furthermore, interfacial alignment has been rarely studied despite its direct relevance and critical importance to charge transport. In this study, we uncover a significantly higher degree of alignment at the top interface of solution coated thin films, using a donor–acceptor conjugated polymer, poly­(diketopyrrolopyrrole-<i>co</i>-thiophene-<i>co</i>-thieno­[3,2-<i>b</i>]­thiophene-<i>co</i>-thiophene) (DPP2T-TT), as the model system. At the molecular level, we observe in-plane π–π stacking anisotropy of up to 4.8 near the top interface with the polymer backbone aligned parallel to the coating direction. The bulk of the film is only weakly aligned with the backbone oriented transverse to coating. At the mesoscale, we observe a well-defined fibril-like morphology at the top interface with the fibril long axis pointing toward the coating direction. Significantly smaller fibrils with poor orientational order are found on the bottom interface, weakly aligned orthogonal to the fibrils on the top interface. The high degree of alignment at the top interface leads to a charge transport anisotropy of up to 5.4 compared to an anisotropy close to 1 on the bottom interface. We attribute the formation of distinct interfacial morphology to the skin-layer formation associated with high Peclet number, which promotes crystallization on the top interface while suppressing it in the bulk. We further infer that the interfacial fibril alignment is driven by the extensional flow on the top interface arisen from increasing solvent evaporation rate closer to the meniscus front